This invention relates to the color stabilization of organic acid anhydrides in general. More particularly, the invention relates to the color stabilization of anhydrides which yield organic acids having between 1 and 16 carbon atoms per molecule upon hydrolysis, of which acetic anhydride, succinic anhydride, maleic anhydride, and phthalic anhydride are exemplary. The stabilization method and compositions therefor provided herein provide heretofore-unseen synergistic effects which provide for reduced stabilization treatment levels and greater color stability over compositions and methods of prior art.
Anhydrides of carboxylic acids are important as raw materials in various industries and such materials are typically available to end users in either solid or molten form, depending upon the particular anhydride and the intended use. Anhydrides of acrylic acid, acetic acid, succinic acid, phthalic acid, and maleic acid are but five examples of such materials, the entire class of which anhydrides is well-known by those skilled in the chemical art.
Most organic acid anhydrides are colorless in appearance immediately after their initial preparation and purification, but are seen to take on or develop a color after being stored at elevated temperatures for extended, or in some cases moderate, periods of time. While the exact reaction mechanism for the development of coloration in such materials varies with the composition in each case, and the inventor hereof does not wish to be bound by any particular theory, it may be surmised that trace quantities of impurities present undergo oxidation, coupling, polymerization, or otherwise react with either themselves or other impurity molecules present, or with the anhydride itself It is also possible that the impurities may function as catalysts for the oxidation of the anhydride or its reaction with itself. Regardless of the mechanism, the development of coloration in such materials is generally indicative of impurities, reactions associated therewith, and/or oxidation.
With the advent of strict quality controls in manufacturing processes, greater attention has in general been paid to the presence of minor impurities in chemical raw materials and other physical attributes such as appearance which were often overlooked in the past. Consistency of quality in raw materials provided to a user has become an increasingly important requisite of the raw material supplier. Therefore, any improvement in the stability of a given raw material is viewed as beneficial to the user of the material and products derived therefrom. Accordingly, a distinct advantage would be gained by the provision of a chemical material or system that may be added to a carboxylic acid anhydride which effectively inhibits chemical reactions occurring within the material when it is maintained at temperatures normally experienced during the usual course of its handling and processing, to the extent that changes in color are minimized over extended periods of time.
The art of stabilization of various organic molecules probably dates back to earlier than the discovery in the 16th century A.D. that ethanol was stabilized against oxidation catalyzed by impurities present in its aqueous solution by the burning of a candle of sulfur in a barrel prior to its being filled with wine. Since that time, the number, type, and actual chemical structure of known organic chemical substances has increased dramatically. The increase in the number of known organic molecules has been attended by an increase in the number of stabilizing additives useful in connection with the known molecules.
One carboxylic acid anhydride of commercial importance is maleic acid anhydride. This material has found widespread use as a raw material for chemical products useful in a variety of industries, including the manufacture of finished fiberglass products, as a graft copolymer with polyisobutylene for use in forming polyisobutylene-succinic anhydride (xe2x80x9cPIBSAxe2x80x9d) based motor oil and spark-ignition fuel dispersants, and as a comonomer for the manufacture of various multi-functional or highly-engineered copolymers to name but a few.
Maleic acid anhydride may be manufactured starting from a wide variety of raw materials, using well-known processes. Typically, this material is derived from the catalytic oxidation of a hydrocarbon involving the passing of a gaseous hydrocarbon over a suitable catalyst material in the presence of oxygen. While the number of hydrocarbons from which maleic acid anhydride may be produced is large, most modern production of maleic anhydride is based upon hydrocarbon feedstocks containing four carbon atoms per molecule. The preferred catalyst systems are those which comprise mixed oxides of the elements vanadium and phosphorus which are prepared by various proprietary and patented processes wherein the oxidation state of the metal is carefully controlled during catalyst preparation and subsequent annealing or other treatments. Exemplary of processes and catalysts useful in the production of maleic acid anhydride are embodied in the following U.S. Pat. Nos. 3,832,359; 4,111,963; 4,149,992; 4,276,222; 4,253,988; 4,304,723; 4,337,174; 4,359,405; 4,501,907; 4,515,973; 4,528,280; 4,562,268; 4,567,158; 4,632,915; 4,670,415; 4,560,674; 4,855,459; 5,137,860; 5,168,090; 5,185,455; 5,275,996; 5,364,824; 5,617,208; 5,631,387; 5,641,722; 5,734,066; and 5,773,382, the entire contents of which are herein incorporated by reference thereto, as well as patents cited in each as references. As produced from these processes, the maleic anhydride may contain by-products of other organic acids or anhydrides, chromogenic bodies, carbon monoxide, carbon dioxide, and water. Crude maleic anhydride prior to purification can be colored other than water white, and while it can be refined to a substantially color-free material, color generally reappears upon storage as hereinabove described. This is due in part at least to the fact that it is convenient from a manufacturer""s standpoint to maintain the temperature at which maleic anhydride is stored in the range of about 60 to 70 degrees centigrade. At such an elevated temperature, many reactions, including colorant-forming reactions between organic molecules occur readily. Therefore, workers have continually sought improved color-stability improvement additive combinations or systems useful in color stabilization of organic acid anhydrides.
In addition to inhibiting reactions which otherwise cause coloration to develop in the anhydride, an additive or system must also not affect the physical properties of the acid anhydride to any extent, and must not in any way interfere with the process(es) or use employed by the end user of the anhydride as a raw material. Therefore, materials or systems which are effective at extremely low concentrations are most desirable.
U.S. Pat. NO. 3,903,117 to Stenseth et al provides for the introduction of a color-stabilizing amount of a treating agent that contains a labile halogen to an organic acid anhydride.
U.S. Pat. No. 3,975,408 to Boyer et al. discloses and claims an improvement for stabilizing the color of dicarboxylic acids, and especially maleic anhydride, which comprises the addition of a chemical agent selected from halides of transition elements, including titanium, zirconium, cobalt, nickel, ruthenium, vanadium, chromium, manganese, mercury, silicon, phosphorous, bismuth, antimony, lead cerium, and sulfur. The level of treatment is between about 0.01 to 1,000 parts per million by weight, based on the total weight of the anhydride.
U.S. Pat. No. 3,985,776 to Samans et al. teaches the stabilization of maleic anhydride through the use of stannous compounds such as stannous chloride and stannous salts of aliphatic monocarboxylic acids. The level of treatment is between about 1 and 2,000 parts per million based on the weight of the maleic anhydride.
U.S. Pat. No. 3,998,854 to Samans et al. sets forth the use of trithiophosphites as stability additives for use in color-stabilization of maleic anhydride, particularly, the trialkyl derivatives of phosphorous acid which are disclosed as effective in this regard at concentrations between 1 and 2,000 parts per million (xe2x80x9cppmxe2x80x9d) based on the total weight of the maleic anhydride.
U.S. Pat. No. 4,062,874 to Schiaraffa et al. teaches the use of the stabilization of maleic acid anhydride using 4,4xe2x80x2-di(hydroxyphenyl)alkanes or with 4-alkylphenols. The concentration level is disclosed to be between 1 and 200 ppm, based on total anhydride weight.
U.S. Pat. No. 4,358,600 to Kuhlmann et al discloses a process for producing maleic anhydride having an improved color property after aging which comprises the metal-chloride catalyzed polymerization of the color bodies present in the crude maleic anhydride prior to its distillation in the rectification process.
U.S. Pat. No. 4,446,264 to Cottman sets forth mixtures of antioxidants produced from reacting maleic anhydride, acid, or esters thereof with thiols, claimed to exhibit synergy when combined with phenolic anti-oxidants, and useful in stabilizing polymers, lubricants, and oils.
U.S. Pat. No. 4,547,539 to Spivac et al. teaches the use of substituted succinic anhydrides as stabilizers for polyolefins and rubbers.
U.S. Pat. No. 4,590,301 to Lim et al. discloses the use of a family of substituted phenols and Quinonoid compounds as being useful as polymerization inhibitors for acrylic and other monomers.
U.S. Pat. No. 5,026,876 provides a process for stabilizing maleic anhydride by adding n-propyl gallate, cuprous chloride and zinc chloride to maleic anhydride. Another embodiment comprises adding tridecyl phosphite to crude maleic anhydride and distilling crude maleic anhydride. Another embodiment comprises adding tridecyl phosphite, n-propyl gallate, cuprous chloride and zinc chloride to maleic anhydride.
U.S. Pat. No. 5,319,106 to Kwon et al. discloses a process for removing residual acrylic acid from crude maleic anhydride prior to the rectification of the maleic anhydride, using phenothiazine as an inhibitor.
While each of the above reference patents, each of which (including patents referenced in each) are herein incorporated by reference thereto, possess varying degrees of desirable characteristics for prevention of color formation in organic acid anhydrides held at temperatures greater than ambient for sustained periods, each has one or more drawbacks associated with it, including relatively high treatment levels, relatively high cost, cumbersome in use, toxicity, etc.
In order to evaluate samples of organic acid anhydrides for color stability, a sample aliquot is placed in a suitable container, such as in a beaker, test tube, or similar vessel having a convenient capacity, usually between about 50 and 250 milliliters with the amount of sample used being in the range of about 25 to 100 milliliters. The vessel and its contents are placed in a location of constant temperature for a prescribed amount of time. One preferred method involves the use of a block of a metallic element or a metallic alloy, into which has been drilled a plurality of holes each having a diameter which is just slightly larger than the outer diameter of the vessel containing the sample that is to be evaluated, to a depth of at least the level of the anhydride in the vessel in which it is contained. The block is brought to the temperature at which it is desired to expose the sample by either external means or, more preferably, an internal heating means such as a resistance coil. Another preferred method involves placing the vessel containing the sample to be tested into a stirred bath of constant temperature, such as an oil bath, for a prescribed time. Following the heat stress treatment, the color of the sample is observed and compared to either the untreated sample, or to a standard chart or the like, and a numerical value is recorded.
At present there is no universally accepted IUPAC or equivalent standard of temperature/time profile under which to subject a given sample of organic anhydride for evaluative purposes relating to color stability. Rather, different countries and regions have adopted such profiles as they individually have seen as being most fit for themselves, based largely on historical empirical observations. For example, in the United States and in parts of Europe samples of maleic anhydride are tested for color stability at 140 degrees centigrade for a two hour time period, with specifications for acceptable color related to the APHA color. Manufacturers in most parts of Asia, however, prefer to expose the anhydride to a temperature in the range of about 180 to 185 degrees centigrade for a time period between about 13 and 15 minutes. It is true in general that the samples of industrially-produced organic acid anhydrides tend to change color more readily under conditions of higher temperatures than when maintained at lower temperatures. In this regard, the higher temperature test may provide quicker results.
Various scales for measurement of the color of organic liquids have been devised over the years. The American Public Health Association (hereinafter xe2x80x9cAPHAxe2x80x9d) has developed a color-matching test which some producers and consumers find convenient. This scale is well-known in the art, and is often used for reporting how far off-color various chemical materials are. The standards are used by simply comparing a liquid""s color to a series of yellow or yellow-brown standards. The results of the test are reported in units known as xe2x80x9cHazenxe2x80x9d units, the use of which is well-known in various arts. Unless otherwise specified, all data reported herein for the color of anhydride materials shall be in Hazen values.
The present invention relates to an organic acid anhydride product having a high degree of discoloration resistance. An organic acid anhydride product according to the invention is formed from the admixture of an organic acid anhydride with additives comprising an acid halide of a carboxylic acid, and a derivative of a hydroxy carboxylic acid having, in a preferred embodiment, between two and sixteen carbon atoms per molecule. The derivative of the hydroxy carboxylic acid may be the acid itself, or salts, esters, or other derivatives of the acid. In another embodiment of the invention, a combination including more than one derivative of a hydroxy carboxylic acid may also be used. Additionally, a transition metal salt which possesses either an organic or an inorganic anion, or a plurality of salts of different transition metals having inorganic and/or organic anions may be optionally included in an anhydride stabilized in accordance with these embodiments according to the invention.
The invention also relates to a process for producing the stabilized organic acid anhydride compositions which includes the admixture of the various components of the composition into an organic anhydride. The anhydride product that results from the admixture is particularly stable with respect to color changes over time, when stored at ambient or elevated temperatures for extended periods. The amount of additives added to the anhydride is in the parts per million range.
To practice the instant invention, one provides an organic acid anhydride, and then adds the synergistic additives combination herein taught to the anhydride in the prescribed amounts, and mixes the resultant mixture until homogeneous using conventional means. The anhydride is preferably in the molten or liquid state to promote uniform mixing, although this is not absolutely necessary. For example, in the case of anhydrides that are solid at ambient temperatures, the solid pellets, chunks, briquettes, or other solid mass of the anhydride may be dusted by a composition comprising the additives taught herein in an effective color-stabilizing amount. Since nearly all consumers of anhydrides normally solid at ambient temperatures render the materials to the liquid state prior to their employment in their end use, a dust coating would be readily incorporated into the mass of the molten substance upon its melting.
Although the compositions taught in the instant invention are described with respect to their ability to inhibit color-forming reactions in organic acid anhydrides, it may be readily anticipated that the compositions may function equally well in other materials and systems where similar products are produced from like starting materials, or are exposed to like storage conditions. Such other materials may include all organic molecules known by those skilled in the art of organic chemistry to undergo an undesirable degree of discoloration.
Any organic anhydride may be stabilized according to the present invention. An organic anhydride is a chemical containing the functional group: 
in which R and Rxe2x80x2 are each independently the same or different hydrocarbyl groups, in accordance with the definition of hydrocarbyl as specified elsewhere herein. Thus acid anhydrides which may be stabilized according to the invention include without limitation those anhydrides which yield organic acids having between 1 and 16 carbon atoms per molecule upon hydrolysis of which acetic anhydride, succinic anhydride, maleic anhydride, and phthalic anhydride are exemplary.